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New Member

backup and alternate port rstp

can anyone tell me what these do, and how they work ?


Re: backup and alternate port rstp

In order to accelerate the spanning tree convergence time the IEEE committee designed a new protocol: 802.1w Rapid Spanning Tree Protocol (RSTP).

Rapid Spanning Tree Protocol (RSTP) is a backward-compatible replacement for Spanning Tree Protocol (802.1d STP). RSTP maintains "alternate" & "backup" ports that become active in case of a failure of the "designated" port, allowing a sub-second convergence time, compared to a 30 second convergence time for STP.

Multiple Spanning Tree (MST) extends RSTP to 802.1q vlan trunks, allowing specific vlans to be blocked on a trunk link without blocking the entire port as STP does. MST requires RSTP.

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New Member

Re: backup and alternate port rstp

in switched LAN RSTP is used to avoid the loops and simultaneously provide redundent paths. to achive this protocol selects one of the best bridge (based on ID) as a ROOT bridge. every other bridge part of that network must and shuold connect to that root bridge only one way to avoid loops. if non-root bridge has two different paths to the ROOT-bridge, then it should discard one of the worst path logically by making it ALTERNATE for that port. if there is any failure in active path this redundent path will be activated.

if two ports are connected in any of the same bridge,it should avoid loop by discarding that path by making the worst port(port ID) as BACKUP .

Re: backup and alternate port rstp

As already mentioned, both backup and alternate ports are put in discarding state, to prevent a bridging loop. Let me just explain quickly why the IEEE introduced a difference between those two kind of discarding ports.

The root port of a bridge is the port connecting the bridge to the root bridge. It continuously receives information from the root. If the root port fails, the bridge must find as soon as possible an alternate path to the root bridge, if one exists.

In STP (any flavor), a port is discarding when it receives some superior information from the root and when it's not root port. That means that a discarding port is in fact signaling an alternate path to the root bridge. Now, here comes the reason for the alternate port vs backup port. If a port is backup (blocked by its own BPDUs, as mentioned in an earlier port), it means that the path to the root that it receives is going through itself. This means that the root information received on the backup port is dependent on the root information received on the root port of the bridge. If the root port information disappears, the root information received on the backup port might also disappear. That's why a backup port cannot be used as a direct replacement for the root port should this latter fails.

On the other hand, an alternate port receives root information from a different bridge. If the root port fails on the bridge, the path advertised on the alternate port does not fail necessarily and can be used as an immediate replacement for the root port.

The alternate ports are thus part of what was called the uplink group in uplinkfast terminology: a group of ports that are eligible to replace the root port in case of failure.